The blood–brain barrier (BBB) and blood–nerve barrier ensure protection of the nervous system but pose a challenge for the treatment of pain since it restricts passage of many therapeutic drugs. Although it is unknown which blood–neural barrier is more relevant, or whether permeabilities are the same for different barriers, we proposed that the inefficiency of thiazolidinedione-type agonists for peroxisome proliferator-activated receptor gamma (PPARɣ) is due to their difficulty in passage through the BBB. We developed a new highly BBB penetrable PPARɣ agonist for the treatment of neuropathic pain, assuming BBB permeability is a rule of thumb to estimate the overall permeability of relevant blood–neural barriers.  As an index of brain penetration, the brain–plasma ratio (Kp) of ELB00824 is 5.13, suggesting very high brain bioavailability, which is 58-fold that of pioglitazone. The series of studies presented here indicate that ELB00824 may be the most potent PPARɣ agonist currently known for acute reduction of neuropathic pain in trigeminal nerve in rat and mouse models. Low-dose PPARɣ agonist, ELB00824 (10 mg/kg), effectively decreased neuropathic hypersensitivity in mice and rats at both acute and chronic time points, a dose 100-fold lower than the effective dose (1000 mg/kg, i.p.) of pioglitazone. Comparisons of ELB00824 alone or in combination with gabapentin or carbamazepine are provided. While PPARɣ agonists used to treat Type 2 diabetes produce several adverse side effects, sub-chronic oral toxicity study provided promising results that ELB00824 does not produce any significant short-term toxicity. The study animals of either sex remained alive and healthy with no significant alteration of body weight long term. Toxicity study results obtained were satisfactory, with no significant alterations in any serum biochemistry parameters.

Effective, non-addictive therapeutics for chronic pain remain a critical need. While there are several potential therapeutics that stimulate anti-inflammatory mechanisms to restore homeostasis in the spinal dorsal horn microenvironment, the effectiveness of drugs for neuropathic pain are still inadequate. The convergence of increasing knowledge about the multi-factorial mechanisms underlying neuropathic pain and the mechanisms of drug action from preclinical studies are providing the ability to create pharmaceuticals with better clinical effectiveness. By targeting and activating the peroxisome proliferator-activated receptor gamma subunit (PPARγ), numerous preclinical studies report pleiotropic effects of thiazolidinediones (TDZ) beyond their intended use of increasing insulin, including their anti-inflammatory, renal, cardioprotective, and oncopreventative effects. Several studies find TDZs reduce pain-related behavioral symptoms, including ongoing secondary hypersensitivity driven by central sensitization. Previous studies find increased PPARγ in the spinal cord and brain regions innervated by incoming afferent nerve endings after the induction of neuropathic pain models. PPARγ agonist treatment provides an effective reduction in pain-related behaviors, including anxiety. Data further suggest that improved brain mitochondrial bioenergetics after PPARγ agonist treatment is a key mechanism for reducing hypersensitivity. This review emphasizes two points relevant for the development of better chronic pain therapies. First, employing neuropathic pain models with chronic duration is critical since they can encompass the continuum of molecular and brain circuitry alterations arising over time when pain persists, providing greater relevance to clinical pain syndromes. Assisting in that effort are preclinical models of chronic trigeminal pain syndromes. Secondly, considering the access to nerve and brain neurons and glia across the blood–brain barrier is important. While many therapies have low brain penetrance, a PPARγ agonist with better brain penetrance, ELB00824, has been developed. Purposeful design and recent comparative testing indicate that ELB00824 is extraordinarily efficient and efficacious. ELB00824 provides greatly improved attenuation of pain-related behaviors, including mechanical hypersensitivity, anxiety, and depression in our chronic trigeminal nerve injury models. Physiochemical properties allowing significant brain access and toxicity testing are discussed.